1. Field of the Invention
The present invention relates generally to an apparatus and method for continuously producing and pelletizing gas hydrates using a dual cylinder and, more particularly, to an apparatus and method for continuously producing and pelletizing high-density gas hydrates containing little water using a squeezing operation of a dual cylinder unit in a reactor.
2. Description of the Related Art
As is well known to those skilled in the art, clathrate hydrates are crystalline compounds in which guest molecules are physically trapped, without chemical bonding, inside a three-dimensional lattice structure formed by hydrogen bonded host molecules. Typically, when the host molecule is water and the guest molecule is a small molecule of gas, such as methane, ethane, propane or carbon dioxide, the clathrate hydrates are called gas hydrates.
Gas hydrates were first discovered in 1810 by Sir Humphry Davy of England. In a Bakerian lecture for The Royal Society of London for Improving Natural Knowledge, he published the fact that when chlorine and water react with each other, compounds having structures similar to ice are formed but the temperature of the compounds is higher than 0° C. In 1823, Michael Faraday discovered the fact that a gas hydrate is formed by reacting one chlorine molecule with ten water molecules. To date, scientific research on gas hydrates as one type of phase change material (PCM) have been regularly conducted. Research into their phase equilibrium, formation/dissociation conditions, coexistence of polycrystals, competitive composition variation in cavities, etc. are representative. In addition, in the microscopic or macroscopic aspect, various detailed research has been being conducted.
About one hundred thirty kinds of guest molecules which can be trapped in host molecules for gas hydrates have been discovered to date. CH4, C2H6, C3H8, CO2, H2, SF6, etc. are representative examples of such guest molecules. Furthermore, a gas hydrate crystalline structure forms a polyhedral cavity because of hydrogen bonded host molecules. According to the kind of gas molecules and the formation conditions thereof, the gas hydrate crystalline structures are classified into a body-centered cubic structure | (s|), a diamond cubic structure ∥ (s∥) and a hexagonal structure H (sH). In s| and s∥, the size of the guest molecule is the critical factor. In sH, the size and shape of the guest molecule are the critical factors.
The most common guest molecule of gas hydrates which are naturally present in deep sea and in permafrost regions is methane. Much attention is being focused on such methane as an environmentally-friendly clean energy resource because little carbon dioxide (CO2) is generated when it burns. In particular, gas hydrates can be used as energy sources which can substitute for fossil fuels. Furthermore, gas hydrates can be applied to natural gas solidification-storage and transfer using the hydrate structure. In addition, gas hydrates can be used in isolation/storage of CO2 for the prevention of global warming. Furthermore, gas hydrates can be used, particularly, in a desalination apparatus, as a technology for separating gaseous or water solutions. As such, gas hydrates are of great utility.
Such gas hydrates are found mainly in regions adjacent to petroleum or natural gas reservoirs or coal seams, or in low temperature and high pressure bathyal deposits, in particular, continental slopes. Furthermore, gas hydrates may be artificially produced. A representative conventional apparatus for producing gas hydrates typically has the structure illustrated in FIG. 1.
FIG. 1 is a schematic view showing a typical apparatus for producing gas hydrates, according to a conventional technique.
The gas hydrate producing apparatus 10 according to the conventional technique includes a water supply unit 1, a gas supply unit 2, a reactor 3 and a discharge unit 5. Water is supplied from the water supply unit 1 into the reactor 3 and gas is supplied from the gas supply unit 2 into the reactor 3. The water and gas react with each other in the reactor 3. Gas hydrates produced in the reactor 3 are discharged from the reactor through the discharge unit 5. The apparatus 10 may further include an agitator 4 to increase the reaction rate between the water and the gas.
In detail, representative examples of such conventional techniques disclosed in documents will be introduced herein below.
Apparatuses or methods of producing gas hydrates were proposed in Japanese Patent Registration No. 3173611, U.S. Pat. No. 5,536,893 and U.S. Pat. No. 6,855,852. These conventional techniques have in common the operation of supplying gas, the operation of supplying water, the operation of producing gas hydrate particles by reacting gas and water with each other, and an agglomerating operation. Some of these techniques further include a recycling gas operation and a cooling operation.
Furthermore, a method of producing hydrates using a water spraying manner was proposed in Japanese Patent Registration No. 3517832. In this technique, water is supplied from a water supply unit into a reactor in such a way as to spray water into the reactor. Therefore, when water is supplied into the reactor, the contact area of water with the gas is increased, thereby enhancing the reaction rate between water and gas.
Another conventional technique was proposed in Japanese Patent Registration No. 4045476, entitled “Apparatus and method of producing gas hydrates”. In this technique, gas is mixed with and dissolved in water to form reaction water. The reaction water flows through a predetermined pipeline. A separate cooling unit cools the pipeline.
Furthermore, another conventional technique was proposed in Japanese Patent Registration No. 3891033, entitled “Apparatus for consecutively producing gas hydrates”. This apparatus includes a rotating drive shaft, blades and a take-out pipe. The rotating drive shaft is installed upright in a reactor in which water and gas react with each other. The blades are provided at positions spaced apart from the center of the rotating drive shaft with respect to the radial direction by a predetermined distance. The blades are disposed such that the surface of reaction water comes into contact with the blades. The blades rotate around the rotating drive shaft. Slurry which gathers around the rotating drive shaft because of the rotation of the blades is discharged below the reactor through the discharge pipe.
In addition, another conventional technique was proposed in Korean Patent Registration No. 0786812, entitled “Gas hydrate producing or dissolving apparatus having constant temperature maintaining tank”. The apparatus according to this technique includes a reaction chamber in which gas hydrates are formed by water and gas reacting with each other or are dissolved, and a constant temperature maintaining tank which maintains the temperature of the water in the reaction chamber at a constant.
However, the apparatuses according to the above-mentioned conventional techniques have in common the following problems.
In the conventional apparatuses, it is difficult to continuously produce gas hydrates. In the case of the apparatuses provided in Japanese Patent Registration No. 3173611, U.S. Pat. No. 5,536,893, U.S. Pat. No. 6,855,852, Japanese Patent Registration No. 3517832 and Korean Patent Registration No. 0786812, several gas hydrates can be produced in a laboratory. However, few concrete studies into a process of extracting gas hydrates from reaction water formed by gas and water have been carried out. Furthermore, in these apparatuses, excessive time and power are required to produce gas hydrates. Therefore, it is almost impossible to continuously produce gas hydrates.
Moreover, in the apparatuses according to the conventional techniques, the process of producing gas hydrates is very long and complex. In particular, the technique disclosed in Japanese Patent Registration No. 4045476 provides the apparatus for consecutively producing gas hydrates, but the process of extracting gas hydrates from a slurry of reaction water is very complex. Furthermore, in all operations of the gas hydrate producing process, the temperatures and pressures of components of the apparatus must be maintained at a constant. Hence, substantially, there are many restrictions in the process of continuously producing gas hydrates. In addition, it is also very difficult to maintain the temperature and pressure of all components at a constant in all of the operations of the process.
Furthermore, because the interior of the reactor must be maintained under high pressure in response to conditions for producing gas hydrates, it is not easy to inject gas into the reactor which is under high pressure. Furthermore, because the reaction rate between gas and water cannot be increased, the gas hydrate production rate cannot be enhanced.